In a natural knee joint, the distal end of the femur articulates against the proximal end of the tibia. The knee joint is supported by various ligaments, including the posterior cruciate ligament (PCL) and the anterior cruciate ligament (ACL). These ligaments stabilize the knee joint while at rest in full extension. Also, these ligaments cooperate to control the complex movements of the knee joint during flexion and extension. The PCL originates medially on the distal femur and attaches to the posterior side of the proximal tibia to resist posterior translation of the tibia relative to the femur. The ACL, on the other hand, originates at the distal femur and attaches to the anterior side of the proximal tibia to resist anterior translation of the tibia relative to the femur.
During flexion and extension of the knee joint, the ligaments of the knee joint work in concert with the meniscus and the geometry of the femur and tibia to effect rotation of the femur about an axis that is offset in a medial direction relative to the center of the knee joint. As a result, the lateral femoral condyle travels along an arcuate path across the proximal tibia, while the medial femoral condyle maintains a relatively central position on the proximal tibia.
Referring to FIGS. 1-7, a natural knee joint is shown in various degrees of flexion and extension.
The knee joint is shown in full extension in FIGS. 1-3. When the knee joint is in extension, femur 10 and tibia 12 are aligned such that, from the side view of FIG. 1, femur 10 and tibia 12 extend along a substantially straight line. In this position, medial and lateral condyles 14, 16, of femur 10 are positioned atop medial and lateral portions 18, 20, of tibial plateau 22, respectively. Ovals 28, 30, of FIG. 2 depict the contact area of medial and lateral femoral condyles 14, 16, respectively, upon tibial plateau 22. As shown, contact areas 28, 30, of medial and lateral femoral condyles 14, 16, are generally centered atop tibial plateau 22. Contact areas 28, 30, of medial and lateral femoral condyles 14, 16, have sufficient length L and width W to provide stability to the knee joint when in extension. Additionally, as shown in FIG. 3, intercondylar eminence 24 of tibial plateau 22 extends proximally into intercondylar notch 26 of femur 10 to provide additional stability to the knee joint. In this position, the ACL (not shown) resists anterior translation of the tibia 12 relative to the femur 10.
The knee joint is shown in mid-flexion in FIGS. 4 and 5. Specifically, the knee joint is bent such that femur 10 is rotated relative to tibia 12 by approximately 45°. As the knee joint moves into mid-flexion, the femur 10 rotates about an axis A that is medially offset from a centerline of the knee joint. Therefore, contact area 30′ of lateral femoral condyle 16 advances posteriorly from full extension to mid-flexion, as shown in FIG. 5 by comparing the contact area 30 in full extension (shown in phantom) with the contact area 30′ in mid-flexion (shown in solid lines). Also, contact area 30′ of lateral femoral condyle 16 advances posteriorly relative to contact area 28′ of medial femoral condyle 14. Additionally, as the knee joint moves into mid-flexion, contact areas 28′, 30′, decrease in length L′ and width W′, because the radius of curvature of medial and lateral femoral condyles 14, 16, decreases posteriorly both in a plane parallel to a sagittal plane (e.g., along length L) and in a coronal plane (e.g., along width W).
The knee joint is shown in a state of increased flexion in FIGS. 6 and 7. Specifically, the knee joint is bent such that femur 10 is rotated relative to tibia 12 by another 45°, with femur 10 and tibia 12 forming an angle of approximately 90° therebetween. As the knee joint continues to bend, the femur 10 continues to rotate about axis A. Therefore, contact area 30″ of lateral femoral condyle 16 advances posteriorly from mid-flexion to full flexion, as shown in FIG. 7 by comparing the contact area 30′ in mid-flexion (shown in phantom) with the contact area 30″ in full flexion (shown in solid lines). Additionally, as the knee joint continues to bend, contact areas 28″, 30″, decrease further in length L″ and width W″. The reduced size of contact areas 28″, 30″, eases the ability for femur 10 to rotate relative to tibia 12 about axis A.
Because the lateral femoral condyle 16 travels further across tibial plateau 22 than medial femoral condyle 14, tibia 12 rotates relative to femur 10 during flexion and extension. As the knee joint flexes from full extension (FIG. 2) to mid-extension (FIG. 5), tibia 12 rotates internally relative to femur 10. Conversely, as the knee joint extends from mid-extension (FIG. 5) to full extension (FIG. 2), tibia 12 rotates externally relative to femur 10. This external rotation of tibia 12 tightens the ligaments of the knee joint and “locks” the knee joint against further rotation to stabilize the knee joint in full extension. This behavior of the knee joint when reaching full extension is known as the “screw home mechanism.”
When a normal knee joint becomes damaged and knee arthroplasty is required, it may be necessary to sacrifice ligaments of the knee joint, including the ACL. However, without the ACL, it may be difficult to recreate the stability and the complex movements of the natural knee joint. For example, without the ACL, the tibia may translate anteriorly relative to the femur. Also, without the ACL, the lateral femoral condyle may not rotate about a medially offset axis.